Chemical-mechanical polishing (CMP) is the process of removing projections and other imperfections from a semiconductor wafer to create a smooth planar surface. The wafer is the basic substrate material in the semiconductor industry for the manufacture of integrated circuits. Wafers are typically created by growing an elongated cylinder or boule of single crystal silicon and then slicing individual wafers from the cylinder. Slicing causes both faces of the wafer to be somewhat rough. Planarization is desirable because the front face of the wafer on which integrated circuitry is to be constructed must be substantially flat in order to facilitate reliable semiconductor junctions with subsequent layers of material applied to the wafer. Composite thin film layers comprising metals for conductors or oxides for insulators must also be made of a uniform thickness if they are to be joined to the semiconductor wafers or to other composite thin film layers.
Planarization is typically completed before performing lithographic processing steps that create integrated circuitry or interconnects on the wafer. Non-planar surfaces result in poor optical resolution of subsequent photolithographic processing steps which in turn hinders high-density features from being adequately printed. If a metallization step height is too large, open circuits will likely be created. Consequently, CMP tools are continually being improved upon with an aim toward controlling wafer planarization.
In a conventional CMP assembly the wafer is secured in a carrier connected to a shaft. The shaft is typically connected to a transporter that moves the carrier between a load or unload station and a position adjacent to a polishing pad. One side of the polishing pad has a polishing surface thereon, and an opposite side is mounted to a rigid platen. Pressure is exerted on a wafer back surface by the carrier in order to press a wafer front surface against the polishing pad. Polishing slurry is introduced onto the polishing surface while the wafer and/or polishing pad are moved in relation to each other by means of motors connected to the shaft and/or platen. One way that the slurry is supplied to the polishing surface is through one or more holes in the polishing pad. The holes in the polishing pad are in communication with a supply source via holes or passageways provided in the platen.
The above combination of chemical and mechanical stress results in removal of material from the wafer front surface in a planar manner. One requisite for removing wafer material at a high rate (“removal rate”) and for forming a wafer with high surface uniformity is a uniform distribution of slurry about the polishing surface. FIGS. 1(A) and 1(B) depict one common polishing pad 10 that has a top surface characterized by a series of grooves 11 that are patterned as concentric arcs. FIG. 1(B) is a magnified view of the region surrounded by a rectangle in FIG. 1(A) for the purpose of better viewing the grooves 11. The grooves 11 shown in FIGS. 1(A) and 1(B) do not exactly represent the actual groove number and curvature for a conventional polishing pad, but are drawn to generally illustrate the conventional polishing pad groove configuration. The grooves 11 facilitate widespread slurry distribution across the polishing pad 10. The grooves 11 terminate at the polishing pad edge, and slurry that is forced off the edge is replaced by a continuing slurry supply.
The main driver in biasing the slurry flow toward the perimeter of a polishing pad is the pressure imbalance from the center to the edge of the pad. Slurry disposed at the pad center will have a highly resistive fluid path when compared to the fluid resistance path at the pad perimeter. The densely distributed grooves 11 in the polishing pad 10 depicted in FIG. 1 would ideally facilitate uniform slurry distribution. However, visualization experiments and virtual fluid modeling have shown that the pattern of concentric arcs causes the center of the pad 10 to be deprived of slurry and the perimeter of the pad 10 to be oversupplied.
One attempt at overcoming uneven slurry distribution across a pad included the addition of perpendicularly intersecting grooves 12 to the polishing pad 10 as depicted in FIG. 1. The grooves 12 were evenly spaced at a pitch of ¼ inch to 1 inch. This X-Y grid of grooves 12 improved the slurry distribution to some extent, although not entirely. The reason for uneven slurry distribution is evident when reviewing the pattern of the arced grooves 11 within a square in the X-Y grid. For example, in an actual polishing pad there are 17 grooves meeting the polishing pad edge and thereby facilitating the evacuation of slurry from the perimeter square segment marked “A.” By comparing this to the square segments marked “B” and “C” where there are, respectively, 10 and 0 grooves facilitating the evacuation of slurry from the segments from the polishing pad edge, it is clear why slurry tends to flow away from certain areas and accumulate in other pad areas. Although the exact relationship between slurry distribution and CMP is not quantified, empirical evidence shows that slurry distribution has a direct impact on wafer non-uniformity and removal rate.
Accordingly, it is desirable to stabilize wafer removal rate during a CMP process and to improve wafer uniformity. In addition, it is desirable to accomplish these goals by providing a polishing pad that facilitates even distribution of slurry over the pad during a CMP process. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.